The present invention relates to a process for recovering hydrocarbon polymer from a solution of such polymer in a solvent by using high temperature to cause a phase separation. More particularly, the invention relates to a process for improving the efficiency of the separation of hydrocarbon polymers from their solution in a solvent.
In the manufacture of such polymers by solution polymerization, a common technique used to separate the polymers from the solution in which they are formed is by steam stripping. In this method, the polymer solution and steam are added to a drum of agitated hot water to flash distill the solvent. As the solvent is removed, the polymer precipitates and is recovered by filtration or screening in the form of wet crumb particles. These particles can then be dried by standard methods such as extrusion.
Because of the rising cost of energy, including steam, the above method has become expensive and various techniques have been explored for reducing the amount of steam and energy required for separation of polymer from the solvents in which they were formed. One such technique is high temperature phase separation. A number of variations of the general technique are known. For example, U.S. Pat. No. 3,553,156 discloses a process in which a solution of ethylene copolymer elastomer in a solvent is heated, while maintaining a sufficient pressure to keep the solution in one liquid phase, to a temperature and pressure at which two liquid phases form, one of these phases being relatively rich in the elastomer. The polymer-rich phase is decanted and the polymer recovered from it by evaporation of the solvent. In this process, the patentees themselves state that it is only necessary that the temperature and pressure are selected so that both phases behave like liquids. They also state, however, that the amount of unreacted monomer, such as propylene, will have its effect on the phase diagram for the polymer-in-solvent system.
Similarly, U.S. Pat. No. 3,496,135 discloses a process in which a polymerization reaction product mixture containing ethylene copolymer is admixed with an anti-degradation agent and, while maintaining sufficient pressure to keep the mixture in one liquid phase, the mixture is heated to a temperature at which two liquid phases form. The two liquid phases are separated by decantation and the solvent from the copolymer-rich phase is evaporated. The anti-degradation agent, e.g., a Lewis base such as furan, dialkyl aluminum alkoxides, etc., are said to protect the copolymer from degradation under the temperature and pressure conditions necessary to produce a phase separation.
Another such process is disclosed in U.S. Pat. No. 3,726,843 in which a solution of an ethylene copolymer is heated to a temperature and pressure sufficient to form two liquid phases, one of which is a copolymer-rich liquid phase. The copolymer-rich liquid phase is separated from the other phase by decantation, and then passed under substantially adiabatic conditions into an intermediate lower pressure zone maintained at a pressure low enough to vaporize sufficient solvent to maintain a solvent vapor phase. The resultant copolymer-rich phase from the intermediate zone is then extruded under substantially adiabatic conditions into a subatmospheric zone at a rate below that at which fines of the copolymer are formed. The latter zone has a temperature and pressure which volatilizes residual solvent in the copolymer-rich phase, and produces an essentially solvent free product.
Such prior art processes are subject to a number of disadvantages. For example, steam is usually used to heat the polymer solutions to a sufficient temperature to obtain the phase separation. Depending upon the solvent used, these temperatures can be quite high, e.g., with hexane as a solvent temperatures of about 250.degree. C. are required. In view of the rising costs of steam energy, such high temperatures are undesirable. Moreover, as noted in U.S. Pat. No. 3,496,135, discussed above, at the temperatures necessary to achieve phase separation, degradation of the ethylene copolymers is possible.